1. Field of the Invention
The present invention is directed to an x-ray tube of the type having an anode whose target area is divided into regions formed of different materials, and having an electron emitter with which at least one focal spot can be generated in each of the regions, so that a plurality of focal spots of different sizes can be generated in at least one region, and to a method for operating such an x-ray tube.
2. Description of the Prior Art
X-ray tubes of the type generally described above are employed, for example, in mammography. The different materials are selected such that x-radiation having different hardnesses can be generated, whereby softer radiation is employed for breasts having a low or medium density and harder radiation is employed for breasts having high density. Moreover, a larger focal spot is employed for normal demands and magnification factors, since this can be more highly loaded and, thus, enables shorter exposure times. A smaller focal spot is employed in case of special demands made on the resolution of the exposures and/or given a need for high magnification factors.
An x-ray tube of the type initially cited is described in a company publication of Machlett dated 1986. This known x-ray tube has a target area divided into two regions formed of different materials, whereby a larger and a smaller focal spot can be generated in each of the regions. This known x-ray tube has two radiation exit windows arranged lying diametrically opposite one another, the exit windows being respectively allocated to the two regions of the anode. This known arrangement has the disadvantage that the x-ray tube must be installed in an x-ray diagnostics apparatus so as to be rotatable by 180.degree., so that the one or the other radiation exit window can be optionally brought into the position suitable for the production of an x-ray exposure, dependent upon which of the two regions of the target area is to be employed. This requires a high structural and financial outlay.
European Application 0 322 260 also discloses an x-ray tube whose target area is divided into two regions formed of different materials, whereby one focal spot can be generated in each of the regions. The focal position is essentially the same for both focal spots, so that it is possible to use both regions without adjustment of the x-ray tube. Since only one focal spot can be generated in each of the two regions, only one focal spot size is available per region.
German OS 22 31 970 discloses an x-ray tube whose target area is composed entirely of the same material. A smaller or a larger focal spot can be optionally generated at the same location in this x-ray tube. Both focal spots lie at the same location.
German PS 29 43 700 discloses an x-ray tube for stereo exposures whose target is likewise entirely composed of the same material. In this x-ray tube, two focal spot pairs having respectively different focal spot spacing are provided, whereby the focal spot pairs have different focal spot sizes. A fifth focal spot that is centrally arranged with respect to the two focal spot pairs is provided for normal exposures. It is indispensable for the proper functioning of this x-ray tube that the focal position of all focal spots be different.